Conventionally, a charging system has been proposed that is capable of charging an electrical storage device mounted in a vehicle by wirelessly receiving charging power from a power source provided outside the vehicle through a resonance method (see Patent Document 1). The charging system of the document includes an electric vehicle and a power supply device, and the electric vehicle includes a secondary self-resonance coil (secondary-side resonance coil), a secondary coil, a rectifier and an electrical storage device. The power supply device includes a high-frequency power driver, a primary coil and a primary self-resonance coil (primary-side resonance coil). The number of turns of the secondary self-resonance coil is set on the basis of voltage of the electrical storage device, the distance between the primary self-resonance coil and the secondary self-resonance coil, and the resonant frequencies of the primary self-resonance coil and the secondary self-resonance coil. The distance between the power supply device and the vehicle varies depending upon the conditions of the vehicle (loading state, air pressure of the tires and the like), and the resulting variation in the distance between the primary self-resonance coil of the power supply device and the secondary self-resonance coil of the vehicle, which brings about a change of resonant frequencies of the primary self-resonance coil and the secondary self-resonance coil. Hence, the document discloses that a variable capacitor is connected to a conductor wire of the secondary self-resonance coil, and when the electrical storage device is charged, charging power to the electrical storage device is calculated on the basis of detection values of a voltage sensor and a current sensor. Further, the document discloses that the capacity of the variable capacitor of the secondary self-resonance coil is adjusted so that the charging power to the electrical storage device becomes the maximum, thereby adjusting LC resonant frequency of the secondary self-resonance coil.